1. FIELD OF THE INVENTION
The present invention relates to an optical element in which an optical waveguide and flat electrodes are mounted on a substrate and an electric field is applied through the flat electrodes to the substrate and the optical waveguide so that the refractive index of a portion of the substrate adjacent to the optical waveguide is varied to interrupt a light beam propagating through the optical waveguide, and more particularly to an optical element which is well adapted for use as an optical modulator for converting an electrical signal into a light signal or an optical switch for permitting or interrupting the propagation of a light signal through an optical fiber or an optical waveguide and which ensures a high light extinction ratio (i.e. ratio of maximum output light intensity to minimum output light intensity), low insertion loss and a high-speed operation.
2. DESCRIPTION OF THE PRIOR ART
Optical modulators and directional optical couplers utilizing the electro-optical effect have been widely used in order to perform a high-speed operation of an optical modulator or optical switch.
Typical optical elements of the type described above are (1) a Mach-Zehnder type thin-film optical modulator (Reference is made to R. G Hunsperger, "Integrated Optics: Theory and Technology", Springer-Verlag, New York, 1982, P. 135), employing single-mode optical waveguides formed into a Y branching path and a Y coupling path, in which the phase difference between two light beams is electrically controlled and (2) a thin-film directional optical coupler having two single-mode optical waveguides spaced apart from each other in which optical energy is synchronously transferred from one guide to the other (ibid., p. 129).
In either element, it is required that the distribution of an electric field of a light beam, the distribution of a refractive index of an optical guide, and the distribution of an applied electric field must be made uniform in the two optical waveguides in order to attain a higher light extinction ratio. However, it has not been easy to increase the light extinction ratios of these optical elements, because of: (1) a complex spatial profile of the refractive index, since an optical waveguide is generally fabricated by the diffusion of impurities; (2) the use of flat electrodes incapable of establishing a uniform electric field within an optical waveguide.
Furthermore, in order to maintain a high light extinction ratio, it is essential that an applied voltage must be controlled precisely and the temperature must be kept constant at a high degree of accuracy. This results in a complicated structure of an optical element.
In addition to the optical elements described above, there has been devised and demonstrated a cutoff type thin-film optical modulator (Reference is made to A. Neyer and W. Sohler, "Appl. Phys. Lett., Vol. 35, (1979) p. 256") in which a refractive index of a straight optical waveguide is electrically controlled so that an incident light beam is guided or radiated into a substrate.
An example of the prior art cutoff type thin-film optical modulators is shown in FIG. 27. In FIG. 27, reference numeral 1 designates a substrate upon which an optical waveguide 2 is formed. Flat electrodes 3A and 3B are placed on both sides of a narrow portion as shown in FIG. 27 and in symmetrical relationship about the axis of the optical waveguide 2. An input prism coupler 5A is placed on the input end of the optical waveguide 2 in order to couple a light beam into the optical waveguide 2, while an output prism coupler 5B is placed on the output end of the optical waveguide 2 in order to receive the light beam propagated to the output end of the optical waveguide 2.
In the prior art cutoff type thin-film optical modulator of the type described above, a guided light beam is converted into a radiated light beam by electrically controlling the difference in refractive indices between the optical waveguide 2 and the substrate 1 underneath the waveguide 2 (in other words, the modulator is operated into the cutoff mode). In such a modulator it is difficult to achieve a sufficiently high light extinction ratio because of the following reasons:
(1) The refractive index of the surface of the optical waveguide 2 is considerably greater than that of the substrate, so that even when the following reason (2) to be described below can be eliminated, a high voltage must be applied in order to decrease the refractive index in order to radiate the guided light beam into the -X-direction.
(2) The intensity of the electric field in the substrate 1 generated by applying a voltage to the flat electrodes 3A and 3B is gradually reduced along the -X-direction, so that a considerably high voltage must be applied in order to reduce the difference in refractive indices between the optical waveguide 2 and the substrate 1 underneath the waveguide.